1. Field of the Invention
The present invention relates to a magnetic memory.
2. Related Background Art
An MRAM (Magnetic Random Access Memory) has a structure where TMR (Tunnel Magnetoresistance) elements are disposed at junctions between bit lines and word lines wired in a lattice form. A normal TMR element has a ferromagnetic layer/nonmagnetic insulating layer/ferromagnetic layer, three-layer structure having a nonmagnetic layer between two ferromagnetic layers. The ferromagnetic layers are normally made of a magnetic transition metal element (Fe, Co, Ni) or an alloy of magnetic transition metal elements (CoFe, CoFeNi, NiFe, or the like), and the nonmagnetic insulating layer is made of Al2O3, MgO, or the like.
One of the ferromagnetic layers (pinned layer) forming the TMR element is fixed in the direction of magnetization, while the other ferromagnetic layer (magnetosensitive layer or free layer) is turned in the direction of magnetization according to an external magnetic field. As a structure of the pinned layer, an exchange coupling type for which an antiferromagnetic layer (FeMn, IrMn, PtMn, NiMn, or the like) is given to one ferromagnetic layer is often used.
Memory information “1” or “0” is prescribed according to a state of the direction of magnetization of two ferromagnets forming the TMR element, that is, depending on whether the direction of magnetization is parallel or antiparallel. When the direction of magnetization of these two ferromagnets is antiparallel, the value of electrical resistance in the thickness direction is larger than that when the direction of magnetization is parallel.
Accordingly, the “1” or “0” information is read out by supplying current in the thickness direction of the TMR element and measuring a resistance value or a current value of the TMR element owing to an MR (magnetic resistance) effect.
It has been conventionally proposed to write “1” or “0” information by turning the direction of magnetization of the magnetosensitive layer of the TMR element by an effect of a magnetic field formed as a result of supplying current to wiring disposed in the vicinity of the TMR element, however, in recent years, a writing method by spin injection has also come to be known.
A nonvolatile random access memory unit described in Patent Document 1 (Japanese Published Unexamined Patent Application No. H11-120758) is constructed by arranging memory cells whose memory state can be switched over by an injection of spin-polarized electrons. The memory cells are concretely formed of, for example, a first ferromagnetic layer and a second ferromagnetic layer stacked via a paramagnetic layer, wherein the direction of magnetization of the first ferromagnetic layer is fixed, and the memory state is switched over by the direction of magnetization of the second ferromagnetic layer. That is, this memory unit has been applied with a propagation theory of a spin-polarized electron flow as a new technology for storing information in magnetic memory cells, and can be assembled in arrays of multi-level metal devices. Memory states in the individual cells correspond to one of the two stable orientations of magnetization in the plane of a ferromagnetic film switching layer. These states can be switched by injecting a spin-polarized electron flow into the storage cells. It has been described in the same document that this makes it possible to greatly increase recording density and thus the reading time can be shortened and power consumption can be reduced.
It has been described in Non-Parent Patent Document 1 (“Spin-torque transfer in batch-fabricated spin-value magnetic nanojunctions,” J. Appl. Phys., May 15, 2003, Vol. 93, Number 10, pp. 6859-6863) that a two-terminal spin injection device could be manufactured by a process using a Co—Cu—Co stack. It has been described in the same document that the stack has been deposited by a sputtering method and electron beam evaporation.
In Non-Parent Document 2 (“Spin Pumping in Ferromagnetic-Metal/Normal-Metal Junctions,” Journal of the Magnetic Society of Japan, 2003, Vol. 27, No. 9, pp. 934-939), a spin injection magnetization reversal has been described. When a spin-polarized current is injected into a ferromagnet, a magnetization precession and reversal occurs, and this phenomenon is called a spin injection magnetization reversal. When a bias current is vertically supplied to the film surface of a multilayer film of a first ferromagnet, a first nonmagnet, a second ferromagnet, and a second nonmagnet, a spin-polarized current flows. The spin-polarized current is a current flow accompanied with a spin flow. When the first and second ferromagnets are different in the direction of magnetization, a spin torque by a spin current works on magnetization of the second ferromagnet. This spin torque serves as a driving force to cause a magnetization motion. The spin injection magnetization reversal can be applied to a writing technique in an MRAM.
When the magnetization reversal using a spin injection is applied to a magnetic memory, a writing current can be reduced.